How are you getting one clock cycle latency?

Here's the lowest latency I can get out of Visual 6502:
http://visual6502.org/JSSim/expert.html ... nc,irq,480

IRQ fired at the end of cycle 6, interrupt sequence starts on cycle 8 (or really on cycle 9, because cycle 8 is just the opcode fetch and discard). The three consecutive stack writes don't even happen until cycle 10!

Hi Rich,

I've probably confused things slightly by talking about interrupt latency as the time from the falling edge of NMI/IRQ to the start of the 7-cycle interrupt sequence. The minimum interrupt latency, including the 7-cycle interrupt sequence, is actually 8 cycles.

You can directly read the minimum latency off the scope plots (which accumulated many measurements over about a minute). The NMI trace shows all shows all possible points at which NMI was seen to fall prior to the trigger event, which was the interrupt sequence (specifically the 3 cycle write):

The left cursor marks the latest point where NMI was seen to fall, just prior to the interrupt sequence starting. The right cursor marks the Phi2 edge just prior to sync going hi. The difference between these cursors is 1.040us, so a tad over one clock cycle. The trace for IRQ looks pretty much identical.

Your Visual6502 example is interesting, in that it shows 1.5 cycles between IRQ going low and the sync at the start of the 7-cycle interrupt sequence. That's a difference of half a cycle compared to what I see in reality. I guess the explanation is that Visual6502 is only working in whole half cycles, and 1.0 cycles is just too late.

Dave

Yes, that makes sense. I'm not sure what internal model Visual6502 uses, but I guess certain quantisation issues will give different results when the timing is as fine as this.

Still, interesting to see some operational differences between NMOS and CMOS variants, and not just instruction set related things. I guess the ADC/SBC instructions in binary mode (which skip the extra cycle at the end for decimal mode flag fixup) would have the same 'problem' (T0 skipped) so maybe it was necessary to find a better solution for this issue.

Yes, sounds like 1+ε is the latency when measured this way, and on visual6502 that looks like 1.5